Many electronic devices commonly used in the electronics industry are sensitive to noise on their power supplies. The degree of this sensitivity often depends upon the particular application the device is designed to implement. Some applications require implementing circuits which are so precise in nature, the circuits cannot function properly in the presence of even small amounts of power supply noise. Such applications include, for example, frequency synthesizers, signal generation (e.g., serial clock recovery), most circuits incorporating voltage controlled oscillators (VCO), and the like. The circuits which implement the above types of applications are particularly sensitive to electronic noise, especially on their power supplies.
For example, in the case of VCOs, typical prior art VCOs generate an oscillating output signal having a specified frequency. In most VCO applications, it is important that the output frequency of the VCO is stable and is a consistent function of the control inputs (e.g., voltage, capacitance, and the like). Where the VCO is used in an application for clock recovery in a serial transmission system, it is important that the output frequency remain stable and constant. Distortion or variation in the VCO output frequency, and hence, the reconstructed clock signal, could lead to sampling errors, lost data, decreased throughput, or other such problems. However, power supply noise can have a very detrimental effect on the VCO's output stability. As a typical VCO draws current from a power supply, the noise with this current (or voltage), or noise from other external devices (e.g., electromagnetic interference), can affect the output frequency. Such noise typically manifests itself as jitter on the rising and falling edges of the output signal, frequency skew in the output signal, or other distortions in the fidelity of the output. Hence, VCOs and other such circuits are considered very noise sensitive devices.
Power supplies for noise sensitive devices are carefully filtered to remove this problematic noise. While higher frequencies are somewhat easier to remove, low frequency noise has proven more difficult and more expensive (e.g., with respect to silicon area or circuit board space) to remove. While this solution is partially effective, as applications have become more complex and as noise sensitivity has increased with increasing levels of integration, power supply noise, particularly low frequency power supply noise, remains problematic. Typical prior art solutions have not proven completely effective.
Prior Art FIG. 1 shows a typical prior art system 100 including a current source for a noise sensitive device. System 100 includes a transistor m1 and a transistor m2. The sources of transistors m1 and m2 are coupled to Vdd (e.g., 3.3v). The drain of transistor m1 is coupled to a current source Iref, which is in turn, coupled to ground. The node between the current source Iref and transistor m1 is coupled to the gates of transistors m1 and m2. A noise sensitive circuit 10 is coupled to receive current from Vdd via transistor m2.
System 100 functions by maintaining a constant current from Vdd to the noise sensitive circuit. The current source Iref is a reference current source having a constant amplitude. Since the node above current source Iref is coupled to the gates of transistors m1 and m2, changes in Vdd should cause corresponding correcting changes in Vgs (e.g., voltage between the gate and the source) for transistors m1 and m2, causing the current flowing through transistor m1 to "mirror" the current flowing through m2. As the voltage level of Vdd changes (e.g., due to noise), the current transmitted to the noise sensitive-circuit via transistor m2 should remain constant. In so doing, noise on Vdd should be isolated from the noise sensitive circuit, allowing the noise sensitive circuit (e.g., a VCO cell of a VCO) to properly function.
The problem with prior art system 100, however, is that the voltage at the drain of transistor m1 does not increase as much as the voltage at the drain of transistor m2. Hence, the current flowing to the noise sensitive circuit cannot precisely mirror the current flowing through transistor m1. This allows noise from Vdd to "leak" through to the noise sensitive circuit and affect its operation.
Thus, what is required is a circuit which solves the power supply noise problems associated with the prior art. What is required is a system capable of provide a noise free current to coupled noise sensitive circuits. What is required is a circuit that prevents noise sensitive devices from being adversely affected by noise in their power supply and exhibits higher power supply noise rejection. Accordingly, the present invention provides a novel solution to the above requirements.